The invention relates to a hearing aid in accordance with the preamble of claim 1.
In such known solutions a signal is derived from the input and/or output signal, which signal is usually representative of the average energy content of the signal over a time determined by respective time elements, and this signal is used for controlling a voltage-controlled filter and, mostly, also a voltage-controlled amplifier.
As is frequently experienced, such common hearing aids do not always lead to the desired success despite careful adaptation. This is to be observed in particular with persons with impaired hearing in the inner ear, which are encountered with rising frequency. The reason for this is mostly caused by the rectifier time constants of the signal rectifiers required for all possible automatic controls and for filters, amplifiers and the like. Common rectifiers usually emit the effective or mean value of the input signal averaged over several periods. The control voltages thus produced and the signal influences thus carried out therefore always trail the actual momentary input signal.
Such hearing aids work perfectly only in one point of the curves for frequency and loudness. In complex input signals such as language signals, for example, it is impossible in this manner to simulate the natural behaviour of the cochlea with respect to time and/or frequency domain, which, moreover, is impaired in inner-ear patients, i.e., it is not equivalent to an ear of usual hearing.
EP-A1-542 710 discloses a circuit for a delay-free signal processing, in which a combinatorial circuit is connected behind a Hilbert circuit which is provided with two all-passes and which supplies at its outputs two signals displaced in their phase relationship by 90.degree., which combinatorial circuit combines the said two signals in accordance with the algorithm ##EQU1## and supplies the output signal of this circuit to a divider as dividend signal, with the divisor input of the divider being connected to an output of an all-pass of the Hilbert circuit.
In this way it is possible to amplify the spectral contrast and to achieve better adaptation possibilities as compared with other hearing aid circuits.
This, however, leads to the disadvantage that the original dynamics of the input signal are lost and that the output signal is provided with no or very low dynamic differences.
Furthermore, from EP-A1-542 711 a circuit has become known with which a frequency transposition is possible in real time.
In this known circuit a Hilbert circuit is also provided, the outputs of which are also connected to a combinatorial circuit which adds these signals according to their amounts or combines them in accordance with the algorithm ##EQU2##
The output of this combinatorial circuit then supplies a signal which corresponds to the momentary amplitude of the input signal, which is only the case in an approximative manner when the addition is made with respect to the amount.
Furthermore, differentiators are connected to the outputs of the Hilbert circuit, the outputs of which are connected to a second combinatorial circuit which add said differentiated signals with respect to their amounts or combine them in accordance with the algorithm as mentioned above. The output of said second combinatorial circuit is connected to the divisor input of a divider whose dividend input is connected to the output of the first combinatorial circuit. The output of the divider supplies a signal representative of the momentary frequency of the input signal, which in the case of an addition of the amounts of the output signals of the Hilbert circuit or its differentiated signals respectively is only the case in an approximative manner.
Owing to the analytic signal representative of the frequency of the intput signal it is possible to achieve a frequency transposition in which the audio intervals are retained substantially, but it is not possible with these signals to adapt sufficiently a hearing aid to the respective requirements of the patient. The reason for the insufficient possibilities of an adaptation are the response and decay times, the compression factors, the transmission characteristics and the like, which in common solutions cannot be changed depending on the input signals.